Pseudomonas aeruginosa is the main pathogen of nosocomial respiratory infections. Its increasing resistance to antibiotics requires the development of new strategies for prevention and control, demanding a better understanding of the modes of transmission and evolutionary dynamics of this bacteria. In patients under invasive mechanical ventilation, the main mode of contamination by Pseudomonas remains debated, with 3 modes of contamination (endogenous, crossed transmission between patients, or environmental origin) of varying importance, mainly depending on the endemic situation of the place of study.